Methods for Producing Sodium Hypochlorite with a Three-Compartment Apparatus Containing a Basic Anolyte

ABSTRACT

An electrochemical method for the production of a chlorine-based oxidant product, such as sodium hypochlorite, is disclosed. The method may potentially be used to produce sodium hypochlorite from sea water or low purity un-softened or NaCl-based salt solutions. The method utilizes alkali cation-conductive ceramic membranes, such as membranes based on NaSICON-type materials, and organic polymer membranes in electrochemical cells to produce sodium hypochlorite. Generally, the electrochemical cell includes three compartments and the first compartment contains an anolyte having a basic pH.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/091,627, filed Aug. 25, 2008, entitled “Three Compartment Apparatusand Method for Producing Sodium Hypochlorite” and U.S. ProvisionalApplication No. 61/120,737, filed Dec. 5, 2008, entitled “ThreeCompartment Electrochemical Process for Production of SodiumHypochlorite,” the entire disclosures of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates in general to electrochemical processesfor the production of a chlorine-based oxidant product. Moreparticularly, the present invention provides an electrochemical methodfor producing a chlorine-based oxidant product, such as sodiumhypochlorite, through the use of a multi-chamber, electrolytic cell thatincludes an anion-conductive membrane, an alkali cation-conductivemembrane, and an anolyte having a basic pH.

BACKGROUND OF THE INVENTION

Some halogen-based oxidants, such as sodium hypochlorite, are commonlyused as disinfecting and bleaching agents. For example, sodiumhypochlorite (NaOCl) is often used to bleach and launder cloth fabrics(e.g., clothing); to disinfect surfaces, such as floors and medicalequipment in hospitals; to sanitize water in wells, waste-watertreatment plants, and other water systems; and for a wide variety ofother applications. In some instances, sodium hypochlorite is marketedas a 3-6 weight % (wt %) solution for use as household bleach. In otherinstances, stronger solutions are marketed for use in the chlorinationof water (e.g., swimming pools) and for use in medical applications. Theexact amount of sodium hypochlorite required for a particularapplication, however, depends on the quantity of water used, the water'schemistry, the water's temperature, the presence or absence of sedimentin the water, contact time, and other similar factors.

Sodium hypochlorite can be produced in a variety of manners. In oneexample of a conventional method, sodium hypochlorite is produced aschlorine is passed into a cold and dilute solution of sodium hydroxide.In another example, sodium hypochlorite is produced through theelectrolysis of brine in a double chamber electrolytic cell. In thisexample, the hydrolysis process produces caustic soda (sodium hydroxide)and chlorine gas, which are mixed together to form sodium hypochlorite.

While the above-mentioned production methods are used to create largeamounts of sodium hypochlorite, such methods are not without theirshortcomings. In one example, some methods for producing sodiumhypochlorite are inefficient and expensive. For instance, some methodsrequire large amounts of electricity to be spent for each unit of sodiumhypochlorite that is produced. In still another example, certainconventional production processes are essentially immobile and thusprevent sodium hypochlorite from being produced at the site where it isto be used. In yet another example, some conventional methods exposecomponents of an electrolytic cell, namely the cathode and anode, torelatively harsh conditions (e.g., scaling and degradation), which tendto shorten the component's lifespan.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for theelectrochemical production of halogen-based oxidant solutions from analkali-salt feed stream using a multi-compartment, electrolytic cellcomprising alkali cation-conductive membrane technologies. The methodsand apparatus of the present invention may provide the capability ofcontinually generating alkali hypohalite from seawater, salt brine, R.O.brine or another alkali halide solution. The alkali metal could includelithium, sodium and potassium and the halogen could be fluorine,chlorine, bromine and iodine.

Generally, the multi-compartment, electrolytic cell comprises a firstcompartment, a second compartment, and a third compartment, which areeach configured to hold an amount of fluid. The first, or anolyte,compartment includes an anode electrode that is positioned to contact ananolyte fluid within that compartment. Similarly, the third, orcatholyte, compartment includes a cathode electrode that is positionedto contact a catholyte fluid within that compartment. The second, ormiddle compartment may be positioned intermediate to, and is in operablecommunication with, both the first (anolyte) and the third (catholyte)compartments. Indeed, in some implementations, the first and the secondcompartments are separated by an anion-conductive membrane (e.g., an ACSmembrane from Astom Corp.), while the second and third compartments areseparated by a cation-conductive membrane (e.g., a NaSICON membrane)that is selective to one type of material (e.g., sodium ions).

The electrolytic cell can be used in any suitable manner that allowssodium hypochlorite or another chlorine-based oxidant product to beproduced through the cell's use. For example, one or more feed streamscan be added to the electrolytic cell, charge can be passed between thecathode and the anode, and the fluids from the various compartments canbe mixed in a variety of ways to form solutions comprising differentconcentrations of sodium hypochlorite.

The feed streams added to the cell can comprise any fluid or fluids thatallow the cell to function properly and to produce sodium hypochloriteor another chlorine-based oxidant product. For example, water, anaqueous sodium chloride solution, an acid containing chlorine (e.g.,hydrochloric acid (HCl) and/or base containing chlorine (sodiumhydroxide) can be added to the first compartment. In another example, anaqueous solution of a chlorine-containing salt or alkali hydroxide(e.g., sodium chloride or sodium hydroxide) can be added to the secondcompartment. In still another example, water, an aqueous solutioncontaining a chlorine-containing salt (e.g., salt brine, reverse osmosis(RO) brine, seawater, tap water containing sodium chloride, etc.),and/or an aqueous solution of an alkali-containing base (e.g., sodiumhydroxide) can be added to the third compartment. In someimplementations, however, an aqueous solution containing between about 1wt % and about 26 wt % of a chlorine-containing salt (e.g., sodiumchloride) is added as the only solution used as a feed stream.

The pH and concentration of the feed stream or streams added to thecompartments can be controlled so the fluid in each compartment has a pHthat allows the cell to function as intended. In other words, the fluidsin the 3 compartments can be tailored to have any suitable pH. Thatsaid, the fluid or anolyte in the first compartment has a pH that isgreater than about 2, and in some instances, greater than about 4.Indeed, in some implementations, the anolyte has a pH of between about 7and about 12. This basic pH of the anolyte may increase the lifespan ofmembranes (e.g., a NaSICON membrane) used in the cell.

With reference to the fluid or electrolyte in the second compartment,the electrolyte has a pH between about 6 and about 14. Additionally, thefluid or catholyte in the third compartment has a pH between about 7 andabout 14.

In some instances, the cell is configured to direct and mix fluid fromone or more compartments into one or more other compartments of thecell. In this manner, the cell may mix the fluids and cause chemicalreactions to occur in desired compartments.

In one example of a suitable method for producing sodium hypochlorite, afeed stream comprising an aqueous sodium chloride solution is added intothe first and third compartments. As current is passed between the anodeand the cathode, sodium chloride in the second compartment is split, andits anions (e.g., Cl⁻) and cations (e.g., Na⁺) are transported throughtheir respective anion- and cation-conducting membranes. Additionally,as current passes between the electrodes, hypochlorous acid andhydrochloric acid (including ions thereof) accumulate in the firstcompartment and sodium hydroxide accumulates in the third compartment.To form sodium hypochlorite, an effluent from the first compartment andan effluent from the third compartment are mixed together and added tothe second (middle) compartment.

In a second example, a feed stream comprising an aqueous sodium chloridesolution is added to the third compartment in which sodium hydroxide isformed. An effluent from the third compartment is then fed from thethird compartment to the second compartment in which sodium chloridesplitting occurs. An effluent from the second compartment is then fedinto the first compartment, in which hypochlorous acid and hydrochloricacid are both produced. As sodium hydroxide from the second compartmentis introduced into the first compartment, the sodium hydroxide reactswith the hypochlorous acid and hydrochloric acid in the firstcompartment to form sodium hypochlorite.

In a final example, a feed stream containing an aqueous sodium chloridesolution is added into the second compartment where sodium chloridesplitting occurs. An effluent from the second compartment is fed intoboth the first compartment and the third compartment. As in the firstand second examples, hypochlorous acid and hydrochloric acid accumulatein the first compartment while sodium hydroxide accumulates in the thirdcompartment. An effluent from the first compartment and an effluent fromthe third compartment are then mixed (e.g., in the first compartment, aseparate vessel, or another suitable location) to form sodiumhypochlorite.

While the described systems and methods have proven particularly usefulfor the production of sodium hypochlorite, the skilled artisan willrecognize that the described methods may be modified to produce one ormore other chlorine-based oxidant products, such as lithium hypochloriteand/or potassium hypochlorite. For example, instead of using sodiumchloride and a NaSICON membrane, the described methods may use anotheralkali-chloride salt (e.g., LiCl, KCl, etc.) solution with a membrane(e.g., a LiSICON membrane, a KSICON membrane, etc.) that is capable oftransporting selected cations from the salt solution into the thirdcompartment.

These features and advantages of the present invention will become morefully apparent from the following description and appended claims, ormay be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

In order that the manner in which the above-recited and other featuresand advantages of the invention are obtained and will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthereof that are illustrated in the appended drawings. Understandingthat the drawings depict only typical embodiments of the invention andare not therefore to be considered to be limiting of its scope, theinvention will be described and explained with additional specificityand detail through the use of the accompanying drawings in which:

FIG. 1 depicts a schematic diagram of a representative embodiment of a3-compartment electrolytic cell;

FIGS. 2 through 4 contain schematic diagrams illustrating somerepresentative embodiments of systems and methods for producing sodiumhypochlorite;

FIG. 5 contains a graph depicting representative results indicating celloperation voltage and pH for the system shown in FIG. 2;

FIG. 6 contains a graph depicting representative results indicating celloperation voltage and pH for the system shown in FIG. 3; and

FIG. 7 contains a graph depicting representative results indicating celloperation voltage for the system shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

Furthermore, the described features, structures, or characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of suitable ion-conducting membranes, feedstreams, methods for mixing fluids within an electrolytic cell, etc., toprovide a thorough understanding of embodiments of the invention. Onehaving ordinary skill in the relevant art will recognize, however, thatthe invention may be practiced without one or more of the specificdetails, or with other methods, components, materials, and so forth. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of theinvention.

The present invention relates to systems and methods for producing ahalogen-based oxidant, such as alkali hypohalide, through the use of amulti-compartment electrolytic cell comprising alkali cation-conductiveand anion-conductive membrane technologies. In one embodiment, the useof the electrolytic cell, both pure and impuresodium-chloride-containing solutions, such as seawater, salt brine, RObrine, tap water comprising sodium chloride, and mixtures thereof, canbe used to produce sodium hypochlorite. To provide a betterunderstanding of the described systems and methods, themulti-compartment, electrolytic cell is first described, followed by adescription of a variety of methods for using the cell.

FIG. 1 illustrates a representative embodiment of the electrolytic cell100. Specifically, FIG. 1 shows that the cell 100 comprises a firstcompartment 102 (an anolyte compartment), a second compartment 104 (amiddle compartment), and a third compartment 106 (a catholytecompartment), which are each configured to contain a quantity of liquid.As illustrated, the first compartment 102 comprises an anode electrode108 that is disposed within that compartment so as to contact an anolytesolution (not shown). Similarly, FIG. 1 shows that the third compartment106 comprises a cathode electrode 110 that is disposed within thatcompartment to contact a catholyte solution (not shown).

The anode electrode can comprise one or more of a variety of materialsthat allow it to evolve chlorine when it is contact with an anolytesolution comprising chlorine ions (e.g., Cl⁻) and when current isrunning between the electrodes. Some non-limiting examples of suitableanode materials comprise dimensionally stabilized anode-platinum ontitanium (DSA), platinized titanium, ruthenium IV dioxide (RuO₂), andother suitable anode materials that are well known in the art.

The cathode electrode can comprise one or more of a variety of suitablematerials that allow it to evolve hydrogen when the current is runbetween the cathode and the electrode, and when the cathode is disposedin a catholyte solution. Some non-limiting examples of suitable cathodematerials include nickel, stainless steel, and other conventionalcathode materials that are stable in a caustic pH.

FIG. 1 illustrates a power supply 116 connected to the anode electrode108 and to the cathode electrode 110 to apply a voltage and currentbetween the two electrodes to drive reactions within the electrolyticcell 100. This power supply can be any known or novel power supplysuitable for use with electrolytic cell.

FIG. 1 also shows the second (middle) compartment 104 is operativelyconnected to the first (anolyte) compartment 102 and the third(catholyte) compartment 106. In particular, FIG. 1 shows the firstcompartment 102 is separated from the second compartment 104 by ananionic membrane 112 that is capable of selectively transporting anions(e.g., Cl⁻) from the second compartment 104 into the first compartment102 during use of the electrolytic cell 100. Some examples of suitableanionic membranes include, but are not limited to, an ACS membrane fromAstom Corp., an AMI membrane from Membranes Int'l, and other known ornovel polymer anion-conductive membranes.

FIG. 1 also shows the second compartment 104 is separated from the thirdcompartment 106 by a cation-conductive membrane 114, which is capable ofselectively transporting specific cations (e.g., Na⁺) from the secondcompartment 104 to the third compartment 106. Some non-limiting examplesof cation-conducting membranes that are suitable for use with thedescribed systems and methods may include any known or novel type ofNaSICON membranes (including, but not limited to, NaSICON-type membranesproduced by Ceramatec Corp.), LiSICON membranes, KSICON membranes, andpolymer cation-conducting membranes (such as NAFION® membranes producedby DuPont). The cation-conductive ceramic membranes may be referred toas MSICON where M is chosen from lithium (Li), sodium (Na), andpotassium (K). MSICON could be generally described as a super ionconducting membrane capable of transporting M⁺ ions where M includesthose elements descried above. In some embodiments in which thechlorine-based oxidant product comprises sodium hypochlorite, thecation-conducting membrane comprises a membrane, such as a NaSICONmembrane, which is capable of selectively transporting sodium ions.

In some specific embodiments, the alkali-ion conducting ceramic membranecompositions comprising NaSICON materials may include at least one ofthe following: materials of general formula M_(1+x)M^(I)₂Si_(x)P_(3−x)O₁₂ where 0≦x≦3, where M is selected from the groupconsisting of Li, Na, K, or Ag, or mixture thereof, and where MI isselected from the group consisting of Zr, Ge, Ti, Sn, or Hf, or mixturesthereof; materials of general formula Na_(1+z)L_(z)Zr_(2−z)P₃O₁₂ where0≦z≦2.0, and where L is selected from the group consisting of Cr, Yb,Er, Dy, Sc, Fe, In, or Y, or mixtures or combinations thereof; materialsof general formula M^(II) ₅RESi₄O₁₂, where M^(II) may be Li, Na, or Ag,or any mixture or combination thereof, and where RE is Y or any rareearth element. In some specific embodiments, the NaSICON materials mayinclude at least one of the following: non-stoichiometric materials,zirconium-deficient (or sodium rich) materials of general formulaNa_(1+x)Zr_(2−x/3)Si_(x)P_(3-x)O_(12−2x/3) where 1.55≦x≦3. In somespecific embodiments, the alkali-ion conducting ceramic membranecompositions comprising NASICON materials may include at least one ofthe following: non-stoichiometric materials, sodium-deficient materialsof general formula Na_(1+x)(A_(y)Zr_(2−y))(Si_(z)P_(3−z))O_(12-δ) whereA is selected from the group consisting of Yb, Er, Dy, Sc, In, or Y, ormixtures or combinations thereof, 1.8≦x≦2.6, 0≦y≦0.2, x<z, and δ isselected to maintain charge neutrality. In some embodiments of thepresent invention it may be advantageous to employ polymericanion-conductive membranes that are substantially impermeable to atleast the solvent components of the anolyte solution in the firstcompartment.

A variety of polymeric anion-conductive membrane materials are known inthe art and would be suitable for constructing the polymericanion-conductive membrane of the present invention, as would beunderstood by one of ordinary skill in the art. In some specificembodiments, the polymeric anion-conductive membranes may include atleast one of the following: NEOSEPTA® anion exchange membranes (fromAstom Corp.) such as grades NEOSEPTA® AM-1, NEOSEPTA® AM-3, NEOSEPTA®AMX, NEOSEPTA® AHA, NEOSEPTA® ACM, NEOSEPTA® ACS, NEOSEPTA® AFN, orNEOSEPTA® AFX; Ionac® MA-3475 or MA-7500 anion membranes (SybronChemicals Inc, NJ); ULTREX™ AMI-7001 anion membrane (Socada LLC, NJ);and PC-SA, PC-SA/HD, PC 100 D, PC 200 D, PC Acid 60, or PC Acid 100anion membranes (PCA GmbH, Germany).

While not shown in FIG. 1, the various compartments of the electrolyticcell may also comprise one or more fluid inlets and outlets. In someembodiments, these inlets and outlets are used to interconnect one ormore of the cell's compartments. By interconnecting the cell'scompartments, effluents from one or more compartments may be mixed withthe contents of one more other compartments in the cell. As used herein,the term effluent and variants thereof may refer to one or more amountsof fluid that are channeled out of one of the electrolytic cell'scompartments. Because the contents of one or more compartments can befed into one or more other compartments, the contents of one compartmentcan be used to change the pH of another compartment and/or to causevarious desired chemical reactions to occur in a specific compartment.

The cell may be used with any suitable feed stream or streams that allowa halogen-based oxidant product (e.g., sodium hypochlorite) to beproduced when the cell is operated. In one example, the feed stream thatis initially added to the first compartment is selected from water, anaqueous solution comprising a salt made from an alkali-chloride salt, ora salt containing an alkali-metal and chlorine (e.g., sodium chloride,lithium chloride, potassium chloride, etc.), and/or hydrochloric acid.In another example, the feed stream that is initially introduced intothe second compartment comprises an aqueous solution that includes asalt containing an alkali-metal and chlorine (e.g., sodium chloride,lithium chloride, potassium chloride, etc.). In still another example,the feed stream that is initially fed into the third compartmentcomprises water, an aqueous solution that includes a salt containing analkali-metal and chlorine (e.g., sodium chloride, lithium chloride,potassium chloride, etc.), and/or an aqueous solution containing analkali base (e.g., sodium hydroxide, lithium hydroxide, potassiumhydroxide, etc.).

In some presently embodiments, the feed streams added to the first,second, and/or third compartments comprise an aqueous solution thatincludes a salt containing an alkali-metal and chlorine (e.g., sodiumchloride, lithium chloride, potassium chloride, etc.). Indeed, where thedescribed methods are used to produce sodium hypochlorite, the feedstreams added to the first, second, and/or third compartments maycomprise an aqueous solution that includes sodium chloride.

Where the feed stream comprises an aqueous sodium chloride solution(e.g., a brine, seawater, tap water solution containing sodium chloride,etc.), the stream may comprise any suitable concentration of sodiumchloride. In some embodiments, the concentration of sodium chloride inthe feed stream is between about 0.2 wt % and about 26 wt %. In otherembodiments, the concentration of sodium chloride in the feed stream isbetween about 5 wt % and about 20 wt %. In still other embodiments, thesodium chloride concentration in the feed stream is between about 6 wt %and about 14 wt % (e.g., about 10 wt %±2 wt %).

In order to better explain how the electrolytic cell can be used toproduce a chlorine-based oxidant, several representative embodiments ofsuitable methods and systems are described with reference to FIGS. 2through 4. While the described systems and methods may be used toproduce other chlorine-based oxidant products, for the sake ofsimplicity, the following examples discuss methods for using theelectrolytic cell to produce sodium hypochlorite.

FIG. 2 depicts a first representative method (Scheme A) for producingsodium hypochlorite. Specifically, FIG. 2 shows that a first feedsolution 10 comprising an aqueous solution of sodium chloride (e.g., anaqueous solution containing between about 1 and about 26 wt % sodiumchloride) is split into a first stream 12 and a second stream 14, whichare respectively fed into the cell's first (anolyte) 102 and the third(catholyte) 106 compartments. While FIG. 2 depicts one embodiment inwhich the first stream 12 and the second stream 14 come directly fromthe same feed solution 10 and, therefore, have the same chemicalcharacteristics, in other embodiments, the first and the second streammay be modified to have chemical characteristics that vary from eachother. For instance, each stream can independently comprise an aqueoussodium chloride solution having a different concentration of sodiumchloride.

As shown in FIG. 2 as well as in Table 1 below, as current from thepower supply 116 passes between the anode 108 and the cathode 110,sodium chloride is split at reaction R-1 in the second (middle)compartment 104 to form chlorine anions (Cl⁻) and sodium cations (Na⁺),which are respectively transported through the anion-conducting membrane112 (e.g., a DSA membrane) and the cation-conducting membrane 114 (e.g.,a NaSICON membrane). In the first compartment 102, at reaction R-2,chlorine gas (Cl₂) is evolved at the anode 108 and chlorine gas andwater in the anolyte react at reaction R-3 to form hypochlorous acid(HOCl) and hydrochloric acid, including ions thereof. In the thirdcompartment 106, another reaction occurs. In particular, at reactionR-4, sodium cations (Na⁺) react with water under the cell's electrolyticcharge to form sodium hydroxide and hydrogen (H²).

As illustrated in FIG. 2, an effluent from the first compartment 102 andan effluent from the third 106 compartment are mixed together (e.g., atreaction R-5) to form sodium hypochlorite, sodium chloride, and water.While this reaction may occur in any suitable location, FIG. 2 showsthat this reaction can occur as the effluent from the first compartment102 is mixed with the effluent from the third compartment 106 within thesecond chamber 104. Finally, R-6 shows that an effluent from the secondcompartment 104 comprises sodium hypochlorite, sodium chloride, hydrogengas, and/or water.

TABLE 1 Chemical Equations for the Reactions in the Cell Shown in FIG. 2Reaction Name/ Example of Suitable Location Reaction Description R-1/Na⁺ + Cl⁻ Transported through membranes Second compartment R-2/Anode 1082Cl⁻ → Cl₂ + 2e⁻ R-3/Anolyte in First Cl₂ + H₂O → HOCl + HCl Compartment102 R-4/Cathode 110 2H₂O + 2e⁻ + 2Na⁺ → 2NaOH + H₂ R-5/Second4 HOCl +HCl + 2NaOH → NaOCl + NaCl + H₂O Compartment 10 R-6 (Overall 2H₂O +2NaCl → NaOCl + NaCl + H₂ + H₂O Reaction)/ Electrolytic Cell 100

It should be noted that throughout this disclosure, the reaction names(e.g., R-1, R-2, . . . Rn) are used for the sake of simplicity and notto identify any particular order in which specific chemical reactionsoccur. Additionally, while examples of suitable locations for thereactions are discussed, the discussed locations are provided forexample only and are not intended to limit the scope of the invention.

FIG. 3 depicts a second embodiment (Scheme B) of a system and method forproducing sodium hypochlorite with the electrolytic cell. Specifically,FIG. 3 illustrates that a sodium chloride feed stream 16 (e.g., asolution with between about 1 and about 26% sodium chloride) flows intothe third compartment 106.

As shown in FIG. 3 and in Table 2, current flowing from the power sourceto the cathode 110 generates hydrogen gas and sodium hydroxide in thethird compartment 106, as shown at reaction R-4. While the hydrogen gascan be handled in any suitable manner, in some embodiments, the hydrogengas produced in the third compartment is vented of collected from thatcompartment.

FIG. 3 shows that an effluent from the third compartment 106 is fed intothe second compartment 104, where sodium chloride splitting occurs atreaction R-1. As shown, an effluent from the second compartment 104 isintroduced into the first compartment 102 where, at reaction R-5, thesodium hydroxide produced in the first compartment reacts with thehypochlorous acid and hydrochloric acid produced in the firstcompartment 102 (according to reaction R-3) to form sodium hypochlorite.As shown at reaction R-6, the final effluent from the first compartment102 comprises sodium hypochlorite, sodium chloride, and/or water.Additionally, as shown below at R-5 in Table 2, the sodium hypochloriteformed in Scheme B can be formed directly in the first compartment 102.

TABLE 2 Chemical Equations for the Reactions in the Cell Shown in FIG. 3Reaction Name/ Example of Suitable Location Reaction Description R-1/Na⁺ + Cl⁻ Transported through membranes Second compartment R-2/Anode 1082Cl⁻ → Cl₂ + 2e⁻ R-3/Anolyte in Cl₂ + H₂O → HOCl + HCl First Compartment102 R-4/ 2H₂O + 2e⁻ + 2Na⁺ → 2NaOH + H₂ Cathode 110 (Third Compartment106) R-5/ HOCl + HCl + 2NaOH → NaOCl + NaCl + H₂O First Compartment 102R-6 (Overall 2H₂O + 2NaCl → NaOCl + NaCl + H₂ + H₂OReaction)/Electrolytic Cell 100

FIG. 4 depicts a third embodiment (Scheme C) of a system and method forproducing sodium hypochlorite with the electrolytic cell. In particular,FIG. 4 shows that a sodium-chloride-containing feed stream 18 (e.g., anaqueous solution with a concentration of between about 1 and about 26 wt% sodium chloride) is fed into the second compartment 104 where sodiumchloride splitting occurs according to reaction R-1. As shown in FIG. 4,an effluent from the second compartment 104 is split and fed into boththe first 102 and the third 104 compartments. In the third compartment,sodium hydroxide and hydrogen gas are produced according to reactionR-4. Moreover, hypochlorous acid and hydrochloric acid are produced inthe first compartment 102 according to reactions R-2 and R-3. FIG. 4shows that the sodium-hydroxide-containing effluent from the thirdcompartment 106 is mixed with the hypochlorous-acid-containing effluentfrom the first compartment 102 according to reaction R-5 to produce thefinal sodium hypochlorite product through the overall reactionillustrated at reaction R-6. This mixing of the effluents from the firstand the third compartments may occur in any suitable location, such aswithin tubing between the compartments, in a vessel outside the cell,etc. For instance, FIG. 4 shows the mixing can occur in the firstcompartment or external to the cell.

TABLE 3 Chemical Equations for the Reactions in the Cell Shown in FIG. 4Reaction Name/ Example of Suitable Location Reaction Description R-1/Na⁺ + Cl⁻ Transported through membranes Second compartment R-2/Anode 1082Cl⁻ → Cl₂ + 2e⁻ R-3/Anolyte in Cl₂ + H₂O → HOCl + HCl First Compartment102 R-4/Catholyte in Third 2H₂O + 2e⁻ + 2Na⁺ → 2NaOH + H₂ Compartment106 R-5/ HOCl + HCl + 2NaOH → NaOCl + NaCl + H₂O First Compartment 104or Separate Vessel R-6 (Overall 2H₂O + 2NaCl → NaOCl + NaCl + H₂ + H₂OReaction)/Electrolytic Cell 100

In the described methods, the pH values of the various compartments canbe controlled in any suitable manner, including, but not limited to:controlling the pH of the various feed streams and/or effluents (e.g.,by controlling the feed streams' and effluents' sodium-chlorideconcentrations, hydroxide concentrations, etc.), changing effluentmixing schemes, and through other methods for controlling pH.

The fluids (e.g., the anolyte, electrolyte, and catholyte) in eachcompartment can comprise any suitable pH. In some embodiments, the pH ofthe anolyte in the first compartment is maintained at a basic pH, or ata pH greater than about 7 (e.g., between about 7 and about 12). Indeed,in some embodiments, the pH of the anolyte is maintained at a pH aboveabout 9 (e.g., between about 9 and about 12). This basic anolyte pH mayserve several purposes, including preventing one or more membranes(e.g., a NaSICON membrane) from being exposed to acidic conditions. Byso doing the basic anolyte can improve the useful lifespan of one ormore of the membranes (e.g., a NaSICON membrane), which may be damagedat lower pHs. Despite the previously mentioned embodiments, in someother embodiments, an acid concentration (e.g., of HOCl and/or HCl)between about 1% and about 10% is also maintained in the cell (e.g., inthe anolyte in the first compartment 102).

In some embodiments, the pH of the fluid (electrolyte) in the secondchamber is maintained at a pH between about 6 and about 13.Additionally, the pH of the fluid (catholyte) in the third chamber ismaintained at a pH between about 7 and about 14. In one embodiment, tocontrol the pHs of the second and third compartments, voltage is appliedto the electrodes to allow a plurality of alkali ions in the secondcompartment to pass through the cation-conductive membrane into thethird compartment. In another embodiment, the concentration of sodiumhydroxide in the third compartment is controlled so as to be maintainedbetween about 1 and about 26 wt %

In the described methods, the power supply can provide any suitablevoltage that allows the cell to produce a chlorine-based oxidantproduct, such as sodium hypochlorite. In some embodiments, the powersupply provides the cell with between about 1 and about 15 volts. Inother embodiments, the power supply causes between about 2 and about 10volts to pass between the anode and cathode. In still other non-limitingembodiments, the power supply causes between about 4 and about 6 voltsto pass between the electrodes.

The power supply can also provide any suitable current density to thecell. Indeed, in some embodiments, the power supply provides betweenabout 20 and about 100 mA/cm². In other embodiments, the power supply isused to provide a current density between about 30 and about 38 mA/cm².In still other embodiments, the power supply provides a current densitybetween about 32 and about 75 mA/cm².

The final effluent produced by the cell may flow at any suitable ratethat provides the final effluent with a suitable concentration of thechlorine-based oxidant product. Different flow rates can alter the pHand/or concentration of the final effluent. The skilled artisan willrecognize that the actual flow rate of the final effluent from the cellcan depend on several factors, including, but not limited cell size,temperature, ambient pressure, etc. In one embodiment, the flow rate ofthe final effluent is between about 2 and about 30 ml/min. In anotherembodiment, the flow rate of the final effluent is between about 5 andabout 20 ml/min. In still another non-limiting embodiment, the flow rateof the final effluent is between about 8 and about 16 ml/min.

Where the described systems and methods are used to produce sodiumhypochlorite, the electrolytic cell may produce any suitableconcentration of sodium hypochlorite. In one example, the describedsystems and methods produce solutions comprising between about 0.5 andabout 15 wt % sodium hypochlorite. In another example, the describedsystems and methods produce solutions comprising between about 0.8 andabout 4 wt % sodium hypochlorite. In still another example, thedescribed systems and methods produce solutions comprising between about1 and about 2.4 wt % sodium hypochlorite.

The described systems and methods can be varied in any suitable manner.For instance, in addition to the described components, theelectrochemical cell may comprise any other suitable component, such asa coolant system, a conventional pH controlling system, etc. Indeed,because the described systems and methods may function best betweenabout 15° and about 30° Celsius, in some embodiments, the described cellis used with a coolant system. In another example, additional chemicalingredients are added to the cell for any suitable purpose (e.g., tomodify fluid pH, to combat scaling on the electrodes, etc.). In stillanother example, effluents from one or more compartments are fed into adesired compartment or compartments at any suitable time (e.g., anysuitable time after the introduction of a feed stream into the cell) andin any suitable amount.

The described systems and methods may have several beneficialcharacteristics. In one example, because the described methods can use abasic anolyte, the useful life of one or more of the membranes (e.g.,the NaSICON membrane) in the cell can be increased over the lifespanthat similar membranes would likely have in acidic environments. Inanother example, the described methods are able to produce usableamounts of sodium hypochlorite with relatively small amounts of energy(e.g., a current density between about 20 and about 50 mA/cm²). In stillanother example, the described methods may use inexpensive ingredients,such as seawater, brine, tap water with sodium chloride, etc. In stillanother example, the described methods may be used to producechlorine-based oxidants, such as sodium hypochlorite, on demand andcontinuously, as desired. In still another example, some embodiments ofthe electrolytic cell may be portable and, thereby, allow sodiumhypochlorite or another chlorine-based oxidant product to be produced atthe site where it will be used.

The following examples are given to illustrate various embodimentswithin the scope of the present invention. These are given by way ofexample only, and it is understood that the following examples are notcomprehensive or exhaustive of the many types of embodiments of thepresent invention that can be prepared in accordance with the presentinvention.

Examples Example 1

In one example of how the electrolytic cell functions, a cell wasprepared and operated according to the systems and methods shown in FIG.2. Specifically, the first and second incoming feed streams 12 and 14were prepared to have a concentration of about 10 wt % sodium chloridein water. After the feed streams were introduced into the cell, the cellwas operated at a current density of about 34 mA/cm². FIG. 5 shows thevoltage applied to the cell was between about 4 and about 6 volts andthe measured pH of the final effluent from the second compartment wasgenerally between about 8.5 and about 13. As a result of thisexperiment, the sodium hypochlorite current efficiency measured for theprocess was about 98.52% and the final concentration of sodiumhypochlorite produced from the cell in R-6 was about 2 g/L.

Example 2

In a second example of how the electrolytic cell functions, a cell wasprepared and operated according to the systems and methods shown in FIG.3. Specifically, the feed stream 16 was prepared to have a concentrationof about 10 wt % sodium chloride in water. After the feed solution wasintroduced into the cell, the cell was operated at a current density ofabout 34 mA/cm². FIG. 6 shows the voltage applied to the cell wasbetween about 5 and about 5.5 volts and the measured pH of the finaleffluent was between about 4 and about 13. As a result of thisexperiment, the cell sodium hypochlorite current efficiency measured forthe process was about 40% and the final concentration of sodiumhypochlorite produced from the cell was about 1 g/L at a solution flowrate of about 12.13 ml/min.

Example 3

In a third example of how the electrolytic cell functions, a cell wasprepared and operated according to the systems and methods shown in FIG.4. Specifically, the feed stream 18 was prepared to have a concentrationof about 10 wt % sodium chloride in water. After the feed solution wasintroduced into the cell, the cell was operated at a current density ofabout 34 mA/cm². FIG. 7 shows the voltage applied to the cell wasbetween about 5 and about 6 volts. As a result of this experiment, thecell sodium hypochlorite current efficiency measured for the process wasabout 60% and the final concentration of sodium hypochlorite producedfrom the cell was about 1.2 g/L at a solution flow rate of about 8ml/min. Additionally, while not shown, the pH of the final effluent fromthe cell was in the range of about 4 to about 14.

The examples provided herein show that various methods (e.g., Schemes A,B, and C) for mixing the effluents from the various compartments havedifferent effects on final sodium hypochlorite concentrations and thecurrent efficiencies of the cell. In particular, the experimentalresults from examples 1 through 3 show that of the schemes presented,Scheme A is the most efficient and produces the highest yield of sodiumhypochlorite while Scheme B is the least efficient and produces thelowest yield of sodium hypochlorite.

While specific embodiments and examples of the present invention havebeen illustrated and described, numerous modifications come to mindwithout significantly departing from the spirit of the invention, andthe scope of protection is only limited by the scope of the accompanyingclaims.

1. An electrolytic cell, comprising: an anolyte compartment holding ananolyte, the anolyte compartment comprising an anode in contact with theanolyte; a catholyte compartment containing a catholyte, the catholytecompartment comprising a cathode in contact with the catholyte; a middlecompartment in operative communication with the anolyte compartment andthe catholyte compartment, the middle compartment further comprising anelectrolyte; a polymeric anion-conducting membrane positioned betweenthe anolyte compartment and the middle compartment; and an alkalication-conductive ceramic membrane selective to one type of cation, thecation-conductive membrane positioned between the middle compartment andthe catholyte compartment, and wherein the anolyte comprises a pHgreater than about 6, such that the electrolytic cell produces ahalogen-based oxidant product.
 2. The electrolytic cell of claim 1,wherein the alkali cation-conductive ceramic membrane comprises a MSICONmembrane selective to M⁺ ions, wherein M comprises one or more oflithium, sodium, and potassium.
 3. The electrolytic cell of claim 1,wherein the alkali cation-conductive ceramic membrane comprises NaSICON.4. The electrolytic cell of claim 1, wherein the electrolyte comprises apH greater than about 5.5.
 5. The electrolytic cell of claim 1, whereinthe anolyte comprises alkali-chloride salt solution.
 6. The electrolyticcell of claim 1, wherein the anolyte comprises a pH between about 6 andabout 12, the electrolyte comprises a pH between about 6 and about 13,and the catholyte comprises a pH between about 5.5 and about
 14. 7. Theelectrolytic cell of claim 1, wherein the halogen-based oxidant productcomprises alkali hypohalite.
 8. The electrolytic cell of claim 7,wherein the catholyte comprise alkali hydroxide and/or alkali halide. 9.An electrolytic cell for producing sodium hypochlorite, comprising: ananolyte compartment holding an anolyte, the anolyte compartmentcomprising an anode in contact with the anolyte; a catholyte compartmentcontaining a catholyte, the catholyte compartment comprising a cathodein contact with the catholyte; a middle compartment in operativecommunication with the anolyte compartment and the catholytecompartment, the middle compartment comprising an electrolyte; apolymeric anion-conducting membrane positioned between the anolytecompartment and the middle compartment; and a NaSICON alkalication-conductive ceramic membrane selective to sodium ions, the NaSICONmembrane positioned between the middle compartment and the catholytecompartment, wherein the anolyte comprises aqueous sodium chloridesolution having a concentration of between about 1 wt % and about 25 wt% sodium chloride, and has a pH greater than about 7, and wherein thecatholyte comprises an aqueous sodium chloride solution having aconcentration of between about 1 wt % and about 25 wt % sodium chloride,and wherein the catholyte further comprises sodium hydroxide at aconcentration of between about 1 wt % and about 26 wt %.
 10. Theelectrolytic cell of claim 9, wherein the anolyte comprises a pH betweenabout 7 and about 12, the electrolyte comprises a pH between about 5.5and about 13, and the catholyte comprises a pH between about 5.5 andabout
 14. 11. A method for creating sodium hypochlorite, the methodcomprising: providing an electrolytic cell having: an anolytecompartment for holding an anolyte, the anolyte compartment comprisingan anode positioned to contact the anolyte; a catholyte compartment forcontaining a catholyte, the catholyte compartment comprising a cathodepositioned to contact the catholyte; a middle compartment for holding anelectrolyte, the middle compartment being in operative communicationwith the anolyte compartment and the catholyte compartment; a polymericanion-conducting membrane positioned between the anolyte compartment andthe middle compartment; and an alkali cation-conductive ceramic membraneselective to one type of material, the cation-conductive membranepositioned between the middle compartment and the catholyte compartment,introducing an aqueous, sodium chloride solution into the electrolyticcell; applying a current between the anode and the cathode; maintaininga pH of the first aqueous fluid greater than about 7; and producing aproduct comprising sodium hypochlorite in the anolyte.
 12. The method ofclaim 11, wherein the aqueous sodium chloride solution is selected frombrine, sea water, another solution comprising water and sodium chloride,and mixtures thereof.
 13. The method of claim 11, further comprisingoperating the electrochemical cell at a temperature between about 5° C.and about 30° C.
 14. The method of claim 11, further comprisingproducing the solution comprising sodium hypochlorite, wherein thesolution comprises sodium hypochlorite at a concentration between about0.1 and about 30 wt %.
 15. The method of claim 11, wherein the productcomprises sodium hypochlorite present at a concentration between about0.1 wt. % and about 15 wt. %.
 16. The method of claim 10, wherein theproduct comprises sodium hypochlorite present at a concentration betweenabout 0.1 wt. % and about 8 wt. %.
 17. The method of claim 11, whereinthe pH of the anolyte is maintained above 7 by addition of sodiumhydroxide from either catholyte or external supply
 18. The method ofclaim 11, wherein the introducing the aqueous sodium chloride solutioncomprises introducing the sodium chloride solution into the anolytecompartment and the catholyte compartment, and wherein the methodfurther comprises feeding effluent from the anolyte in the anolytecompartment and effluent from the catholyte in the catholyte compartmentinto the middle compartment.
 19. The method of claim 11, wherein theintroducing the aqueous sodium chloride solution comprises introducingthe sodium chloride solution into the catholyte compartment, and whereinthe method further comprises: feeding effluent from the catholyte in thecatholyte compartment into the middle compartment; and feeding effluentfrom the electrolyte in the middle compartment into the anolytecompartment.
 20. The method of claim 11, wherein the introducing theaqueous sodium chloride solution comprises introducing the sodiumchloride solution into the middle compartment, and wherein the methodfurther comprises: feeding an effluent from the electrolyte in themiddle compartment into the anolyte compartment and the catholytecompartment; and mixing an effluent from the anolyte compartment and aneffluent from the catholyte compartment to form the solution comprisingsodium hypochlorite.
 21. The method of claim 11, further comprising:maintaining a pH of the anolyte between about 7 and about 12;maintaining a pH of the electrolyte between about 5.5 and about 13; andmaintaining a pH of the catholyte between about 5.5 and about
 14. 22.The method of claim 21, wherein the usage of anolyte of pH above about5.5 will prevent precipitation of scale forming salts present in theelectrolyte onto the cationic membrane.
 23. The method of claim 11,wherein the aqueous sodium chloride solution comprises sodium chlorideat a concentration between about 1% and about 25%.